The agency is asking communities to set up dedicated bus lanes, giving new life to an approach that until recently hasn’t been widely used on American roads. Using either painted lines, orange cones, or other separators, the idea is to empty travel lanes or parking lanes of other traffic and give buses clear passage.

Buses are the MBTA’s least reliable transit mode, typically arriving late at their destinations around 30 percent of the time.

But MBTA buses lopped nearly 20 percent off their travel time through a one-mile section of downtown Everett after the city banned morning parking along one side of Broadway last year. Now, after prodding from the MBTA and local transit activists, Boston is also plotting bus lanes on heavily trafficked roads, including Washington Street in Roslindale. Somerville, Arlington, and Watertown have expressed interest in them, too.

“If you ever wanted an advertisement to get people out of their single-occupancy vehicle, it’s having a bus beside them going three times the speed they are,” said Monica Tibbits-Nutt, an MBTA board member who has aggressively called for cities to reserve more street space for buses. “And I think our bus users deserve it. The amount of people who have decided to live without a car, or to leave a car at home, that’s huge. And they have been ignored.”

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For state transit planners, who are considering several strategies to boost bus service, exclusive lanes are low cost and relatively straightforward, but they require assistance from cities that own the roads. And the bus riders’ gain will come at the expense of motorists, who will suffer through even more congestion or lose on-street parking. The issue is a collision of three timeless truths of Boston transportation: the T is unreliable, traffic is bad, and parking is precious.

“If they had to give up their parking places in front of their homes, I’d see that as a problem. If they do have to give them up, where do they go?” said Terry Fitzgerald, owner of Centre Cut Salon and Spa in Roslindale Square. “Parking has always been a problem and taking away more parking problems is just adding to it.”

But Gene Carlson, a manager at DB&S Lumber at the Forest Hills end of Washington Street, said removing parking from the street should make it easier for his suppliers and customers in large trucks to access his yard. DB&S has off-street parking, but Carlson worried that customers of other nearby businesses that do not may try to use his lot if the bus lane is installed.

An effort to create a bus-only lane between Roxbury and Mattapan in 2009 collapsed amid public dissent. In Roslindale, Boston officials say they will first test how a bus lane will work on narrow Washington Street, which has one lane of travel in each direction, parking on both sides, and bumper-to-bumper traffic often during the week.

By the end of the year the city will run a short experiment banning parking on the inbound side of Washington Street during the morning commute, since nearly 60 percent of all travelers in that direction are on a bus. If all goes according to plan, the idea would then get a longer run during the spring, said Boston transportation commissioner Gina Fiandaca.

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Boston officials are also exploring a bus-only route between North Station and the Seaport District, though those plans are far less developed. The city has already designed a bus-only lane for the new North Washington Street Bridge, which will be built over the next five years. And its long-term plans call for more lanes on other high-ridership roads, possibly from Oak Square to Commonwealth Avenue or along Massachusetts Avenue.

The issue is a collision of three timeless truths of Boston: the T is unreliable, traffic is bad, and parking is precious.

In Roslindale, bus lane advocates argue there will still be room for residents along Washington Street to park, citing research showing that only 55 percent of the spaces are taken by 6 a.m., with the rest filling in later from drivers outside the neighborhood.

When Everett banned morning parking on Broadway in December, the city deployed enforcement officers who were quick to ticket or tow wayward parkers, to condition them that the lane is now off-limits.

Everett officials also acknowledged they rolled out the bus lane quickly, with limited debate, saying angry motorists may have otherwise tried to block it.

“It would have been more drawn out and more difficult” if the city had a lengthier review, said Jay Monty, Everett transportation planner. “Folks who use the buses often aren’t the most vocal residents in the community.”

In Boston the Silver Line has run in a painted bus lane for years along Washington Street, though cars often drift into it. San Francisco has had them for decades, they’re common in Europe, and Seattle, Cleveland, New York, and Baltimore have implemented them in recent years.

The T is hoping Washington Street in Roslindale will be the first of many new bus lanes in the Boston area. Steve Poftak, another board member, suggested the T hire a specialist to work with municipal governments to implement the lanes. And Luis Ramirez, the agency’s general manager, said he plans on “spending a lot of my time” on bus lanes, because they could have a “huge impact.”

The Barr Foundation, the Boston philanthropy that focuses on environmental issues, is offering local communities grants of up to $100,000 to develop ideas for bus lanes and other ways to improve bus service, and is expected to issue awards in December. Arlington and Watertown are among the applicants. Somerville is planning a short bus-only lane on part of Prospect Street near Union Square that could be in place by the end of 2017, and another on Broadway in 2018.

Across the country, urban planners see improving bus service as a cheaper and more flexible way to boost transit, compared to building new rail infrastructure — with bus lanes a key tactic. Some transit advocates say Boston to this point has been slow to catch on.

“That’s definitely a reputation Boston is claiming in transportation circles around the country — they’re slow to innovate,” said Jon Orcutt, an advocacy director with the national group Transit Center. “If Everett is leading the region on bus lanes, or if you have a fraction of the bus lanes Baltimore does, then that’s a sign.”